In the operation of a well, there may be any number of processes which may act to reduce production from the well. Initial drilling processes can create significant amounts of debris, including rock particulates, rock dust and oil mist. In addition, drilling muds and fluids may contain chemicals which can reduce the ability of the formation to produce fluids by reacting with the formation and/or formation fluids to produce precipitates and/or scale. Furthermore, some fluids may also cause clays within the formation to swell, further blocking the formation's ability to flow. The use of fluid loss control fluids may result in filter cake invading the near wellbore area, which could also decrease the formation near wellbore permeability. Over time, additional processes may act to allow water to imbibe into the formation, and/or asphaltenes and paraffins may deposit in the near wellbore area. For example, a well may be shut down for maintenance operations, such as the replacement of tubing. During this shut down, water may creep into the well and/or near-wellbore formation. Additionally, as formation pressure naturally decreases over time, the formation may no longer have sufficient pressure to drive water from the micro cleat system of the formation. Any one of these processes may act to decrease near wellbore permeability and production.
While wells may generally be drilled vertically, in some applications, it may be desirable to steer the wellbore away from vertical, or a wellbore may unintentionally deviate from vertical. It is possible to drill a well in which one or more portions of the wellbore travel horizontally or even such that they are angled up towards the surface. These wells with at least partially non-vertical wellbores are known as deviated or horizontal wells, and are frequently employed with formations which have low natural pressure as this technique increases wellbore exposure to the hydrocarbon-bearing formation. It is also possible to create multiple wellbore segments extending off a main horizontal wellbore. These multiple segments may comprise lateral segments or may form a fishbone-like structure. Furthermore, a vacuum may be employed where the formation pressure is insufficient for economic production. Regardless of the physical characteristics of the well, when the formation pressure or natural driving force is low, wells are particularly susceptible to the problems associated with deposits, rock dust/drilling fluids becoming impacted on the rock face, and/or imbibed water.
Furthermore, degradation may occur regardless of the manner in which the well is completed. Depending on the formation being drilled into and other factors known in the art, it may be desirable to insert a casing into the wellbore. In situations where casing is inserted into the entire wellbore, the well is known as a cased well. In contrast, if no casing is used, the well is known as an open hole well; and, if only a portion of the wellbore is cased, the well may be known as a partially cased hole or partially open hole. When pipe is run into an open hole section and not cemented in place it is called a liner and the well an open hole completion with liner. In some instances the liner may later be pulled or removed for various reasons. Again, regardless of whether a casing or liner is used, one or more of the previously described processes may act to reduce production.
For some wells, it may be desirable to increase formation flow by fracturing the fluid or gas bearing formation. One fracturing method involves the introduction of a fracturing fluid into the formation at high pressure such that cracks in the rock or fractures within the formation are caused to form. These fractures may be effective in increasing the permeability of the formation, and may bypass wellbore damage such as skin damage in the near wellbore area. In some instances a proppant such as natural sand, or engineered products such as coated sand or sintered bauxite may be used. The proppant may be mixed with the fracturing fluid so that following injection of the fracturing fluid, the proppant may be left in the created fractures, holding them open so that permeability is not lost. However, the use of fracturing fluid itself may adversely affect production as the fluid may act to block pores in the formation.
In yet other wells, it may be desirable to increase the surface area of the wellbore, as this may provide additional paths for fluid or gas to migrate from the formation to the wellbore. This additional surface area may be created by forming slots or other contours in the surface of the wellbore. However, again, care must be taken to ensure that in the process of creating the slots, additional debris is not introduced such that it could act to block the formation and hinder production.
In coal bed methane wells, methane is produced from coal formations. During coal mining operations, the presence of the methane is a hazard and it is desirable to degasify, or to remove as much of the methane from the coal formation as possible prior to coal mining operations. It is known to remove the methane from the coal formations through the use of wells. Of course, a methane well may be drilled into a coal bed formation not necessarily for the purpose of degasification, but for the purpose of extracting the methane.
However, as with other types of gas wells, production from a well drilled in a coal bed methane formation may be reduced due to water flooding or due to a buildup of paraffin or undesirable oil within the near-wellbore formation. The source of the water may be either natural, such as natural loading, or may be the result of well operations such as fracturing techniques, casing leaks or, as noted above, water may creep into the well and/or near-wellbore formation during well shut downs. Regardless of its source, the introduction of water into the coal formation may reduce the formation's gas permeability either by blocking gas flow paths, or through the swelling of formation clays.
Thus, there still remains a need for methods and compositions for cleaning wellbore and near-wellbore areas from damage related to drilling, work over operations and natural degradation of the wellbore from production, especially in low pressure formations. There is an additional need to perform cleaning in a manner such that an operator may precisely control the location of the cleaning. There is also a need for methods and apparatus suitable for cleaning a wellbore casing. There is an additional need for methods, apparatus and compositions suitable for increasing gas production in a coal bed methane formation by, for example, restoring the relative gas permeability of the coal bed methane formation. There is also a need for methods, apparatus and compositions suitable for removing water from a coal bed methane formation. Furthermore, there is a need for methods, apparatus and compositions used in the slotting and fracturing of a formation which leave substantially clean slots and/or fractures in the wellbore and near wellbore areas.